Optical parametric oscillators (OPOs) provide an efficient way of converting short-wavelength electromagnetic radiation from coherent-light sources to long wavelengths, while also adding the capability to broadly tune the output wavelength. In general, an OPO system principally includes a short-wavelength laser source and an optical resonator (resonant optical cavity) containing a nonlinear crystal. In some embodiments, additional components include mode-matching optics and an optical isolator.
In general, the OPO operates with three overlapping light beams—an input pump beam having the shortest wavelength (typically, this is coherent light from a laser), and two longer-wavelength beams generated in the OPO called the signal beam and the idler beam. By convention, the shorter-wavelength beam is called the signal beam, and the longer-wavelength beam is called the idler beam. Depending on the application, either the signal beam or the idler beam, or both, will be the output light utilized by other components. The energy of photons in the pump beam (proportional to 1/wavelength) will equal the sum of the energy of photons in the signal beam plus the energy of photons in the idler beam. The pump beam (i.e., excitation light from the short-wavelength laser source) is focused, using the mode-matching optics, through the optical isolator and into the resonant optical cavity, passing through the nonlinear crystal. Parametric fluorescence generated within the nonlinear material is circulated within the resonant cavity and experiences optical gain. When the OPO is excited by a pump-power-per-unit-area above a certain threshold, oscillation occurs, and efficient conversion of pump photons to signal and idler photons occurs. Different configurations of OPOs are possible. Variables include the wavelengths which are resonant within the optical cavity (pump and/or signal and/or idler) and the type of resonator (ring versus linear).
U.S. Pat. No. 6,654,392 issued Nov. 25, 2003 to Arbore et al. entitled “QUASI-MONOLITHIC TUNABLE OPTICAL RESONATOR,” which is hereby incorporated herein by reference, describes an optical resonator having a piezoelectric element attached to a quasi-monolithic structure that defines an optical path. Mirrors attached to the structure deflect light along the optical path. The piezoelectric element controllably strains the quasi-monolithic structure to change a length of the optical path by about 1 micron. A first feedback loop coupled to the piezoelectric element provides fine control over the cavity length. The resonator may include a thermally actuated spacer attached to the cavity and a mirror attached to the spacer. The thermally actuated spacer adjusts the cavity length by up to about 20 microns.
A monolithic resonator typically includes a single block of transparent material having reflecting facets that serve as the mirrors. Usually, the material is strained by changing its temperature. U.S. Pat. No. 4,829,532 issued May 9, 1989 to Kane, which is hereby incorporated herein by reference, describes an alternative where the optical path length of a monolithic oscillator can be adjusted by a piezoelectric element mounted to uniformly strain the entire block in a plane parallel to the plane of the optical path.
After tuning by one free-spectral range of a conventional OPO cavity, the OPO frequency will discontinuously jump by one or more longitudinal-mode spacing of the cavity (this discontinuous jump is called a mode hop). In view of shortcomings in such conventional devices, there is a need for devices that can provide continuous, mode-hop-free wavelength tuning by hundreds of GHz using a straightforward, reliable mechanism, while also supplying other desirable operating characteristics including linear polarization, a power level of several watts, narrow bandwidth, and/or diffraction-limited beam quality.